Lithographic patterning techniques have been employed in conventional fabrication of microelectronic devices, including thin film transistor (TFT) arrays for flat panel application. Conventional photoresist lithographic techniques applied to microfabrication have proved capable of defining structures and forming regions of material on a substrate to within dimensions of about 100 nm.
Based on a printing model, the lithographic process forms a pattern of areas that are either receptive or repellent (non-receptive) to a coating (such as ink) or to some other treatment. Conventional photolithography requires a small number of basic steps, with variations according to the materials used and other factors. A typical sequence is as follows:                (i) wet coating of a positive-working or negative-working photoresist (such as by spin-coating);        (ii) prebake of the photoresist;        (iii) forming the pattern by exposure to some form of electromagnetic radiation through an overlay mask using an optical mask aligner;        (iv) curing of the masked pattern, such as by postbake; and        (v) removal of the uncured portion, using a liquid etchant.        
Following subsequent coating or treatment of the surface, the protective photoresist pattern can then itself be removed.
Steps (i)–(v) may be performed in air, such as in a clean room environment, and are typically performed using separate pieces of equipment. Alternately, one or more steps, such as coating deposition, may be performed in vacuum. Because of the very different nature of processes carried out in each of these steps, it would be difficult and costly to combine steps (i)–(v) in any type of automated, continuous fabrication system or apparatus.
Considerable effort has been expended to improve upon conventional methods as listed in steps (i)–(v) above in order to achieve better dimensional resolution, lower cost, and eliminate the use of chemicals such as etchants. One improvement of particular benefit has been the refinement of plasma etching techniques that eliminate the need for liquid etchants. With reference to step (v) listed above, the use of plasma etching is an enabler for performing microlithographic fabrication in a dry environment.
As is well known to those skilled in the microlithographic art, conventional photoresist materials follow “reciprocity law,” responding to the total exposure received, the integral of illumination over time. Among disadvantages of conventional photoresist use is the requirement for careful control of ambient illumination until curing is complete. Conventional photoresists are typically exposed with light in the UV portion of the spectrum, where photon energy is particularly high. Examples of photoresists used microfabrication of semiconductor components are given in U.S. Pat. No. 6,787,283 (Aoai et al.)
Due to this response characteristic and other shortcomings of photoresists, another type of resist material, namely thermoresist material, has proved to be more suitable for microlithographic fabrication. Thermoresist responds to heat energy rather than to accumulated exposure over time. Most thermoresists respond to radiation in the IR and near-IR range and may be casually termed “IR resists.” However, there can also be alternative thermoresists in the UV region, as described, for example, in U.S. Pat. No. 6,136,509 (Gelbart).
The use of thermoresist offers advantages for providing a dry process alternative to conventional coating (step (i) given above). Moreover, where a thermoresist pattern can be applied directly to a substrate using highly focused radiant energy, the need for masks is eliminated or minimized and both pre-bake and curing processes (steps (ii) and (iv) above) may no longer apply.
Recognizing these advantages, a number of patent disclosures have proposed thermoresist application using laser energy, including the following:                U.S. Pat. No. 5,858,607 (Burberry et al.) discloses a method for directly transferring patterning material from a donor sheet to a lithographic printing plate. A hydrophilic lithographic printing support such as aluminum or coated polyester is overlaid with a coated donor sheet. The donor sheet contains a transfer layer containing a material that absorbs laser radiation and a polymeric binder having reoccurring units of the following units of the following formula:        
                Wherein R1 represents cyano, isocyanate, azide, sulfonyl, nitro, phosphoric, phosphonyl, hetroaryl, or R2 is Hydrogen, alkyl or from the same list as R1 and a receiver element consisting of a support having a hydrophilic surface such that upon imagewise heating the binder is transferred to the hydrophilic receiver surface. The assemblage is image wise exposed with a high intensity laser beam that transfers binder to receiver. The transfer requires relatively low exposure, with no chemical or solution processing of the plate and no post-bake or other post processing.        
U.S. Pat. No. 6,855,384 (Nirmal et al.) discloses a process for patterning a light emitting polymer, forming the emissive layer of an organic electroluminescent device. The Nirmal et al. '384 process provides a transfer donor sheet, bringing the donor sheet into close proximity with a receptor substrate, and selectively thermally transferring the transfer layer from the donor to the receptor. The donor sheet includes a substrate and a transfer layer that includes a blend of a light emitting polymer and an additive. The additive can be selected to promote high fidelity thermal transfer of the transfer layer. U.S. Patent Application Publication No. 2005/0074705 (Toyoda) discloses adhesive transfer of a resist material from a donor sheet to a substrate. In the Toyoda '74705 method, a layer of resist material is fused or melted onto the substrate by means of an irradiating beam of energy. The unfused donor material is then lifted off, leaving the resist pattern adhered to the substrate surface.
Advantageously, the methods of the Burberry et al. '607, Nirmal et al. '384, and Toyoda '74705 disclosures provide a dry process, eliminating any requirement to coat the substrate initially with uncured resist material, eliminating or reducing masking requirements, and allowing the use of plasma etching techniques. However, in spite of these advantages, some performance drawbacks remain.
Adhesive transfer, used in the methods of each of these disclosures, has inherent limitations for maintaining precise tolerances. To illustrate this, FIGS. 1 and 2 represent the adhesive transfer process in simplified schematic form, showing side views (not to scale). Referring to FIG. 1, in an adhesive transfer patterning apparatus 10, a donor sheet 70 having a transfer layer 68, such as a layer of thermoresist material, on a support 72 is placed directly against a substrate 18. A laser beam 26, or some other form of highly focused radiant energy, is applied to donor sheet 70. In the immediate area where laser beam 26 is incident, a photothermal conversion takes place, melting a corresponding portion of transfer layer 68 and thus adhering this portion to the surface of substrate 18. Adhesive transfer is sometimes suitably termed “melt” transfer.
FIG. 2 shows a familiar problem with adhesive transfer. This problem occurs as donor sheet 70 is pulled away from substrate 18. As shown more clearly in the magnified area labeled Q, a feature 74 is formed wherever some portion of transfer layer 68 is adhered to the surface of substrate 18. Here, feature 74 is a portion of the thermoresist pattern, with transfer layer 68 being a thermoresist material. Ideally, at the edge of feature 74 or other structure of resist material, a clean separation occurs between that portion of the resist donor that is intended to stick to substrate 18 and the other portion that is lifted off and “torn away” from adhered feature 74. One of two problems is possible, however, as illustrated in FIG. 2. First, as shown by an excess portion 76 in FIG. 2, a portion of the un-adhered resist donor may not separate cleanly when overlaying donor sheet 70 is removed. Excess portion 76 may even be inadvertently adhered to the substrate 18 surface, such as through overheating, or may be torn from the balance of transfer layer 68. The existence of excess portion 76 may lead to jagged edges of circuit traces, for example.
A second possible problem relates to a portion of the adhered resist transfer layer 68 that is not perfectly affixed to substrate 18, due to some slight surface imperfection for example, or due to excessive thickness or strength of the surrounding un-adhered resist donor on resist transfer layer 68. In FIG. 2, a torn portion 78 is lifted off along with donor sheet 70. Where torn portion 78 is small, there may be no perceptible effects. However, in some cases, this effect could cause jagged edges of surface features where excessive resist material has been removed.
Various measures can be taken to lessen the likelihood of torn portions and to improve overall adhesion bonding. For example, to counter such effects and obtain clean separation from the donor sheet, the Toyoda '74705 disclosure even suggests the addition of a mold-releasing lubricant or other agent in the donor sheet structure. But because it is not possible to obtain perfect separation between adhered and un-adhered portions of a pattern, adhesive transfer, as proposed in the Burberry et al. '607, Nirmal et al. '384, and Toyoda '74705 disclosures, suffers from inherent problems in maintaining precision edge definition. This, in turn, limits the dimensional resolution that can be obtained for a resist pattern formed using adhesion bonding methods.
A further, significant disadvantage of adhesive transfer relates to overall energy level requirements. When a donor sheet is in flush contact with a receiver substrate, the laser spot necessarily loses some amount of heat through thermal diffusion. This effect, in turn, requires the use of higher exposures to effect any physical change needed for melting and adhesion. Thermal diffusion can be particularly troublesome when the receiving substrate is a metal surface with a high thermal conductivity.
Yet other disadvantages of adhesive transfer relate to the need for intimate, planar surface contact between donor and receiver substrate. Accuracy and high resolution require that the donor be in contact with the receiver during adhesive transfer. The presence of any type of surface features on the receiver surface tends to separate the donor from the receiver surface, resulting in less-than-ideal bonding conditions for precision transfer using adhesive transfer techniques. Similarly, dust or dirt particles, inevitable even in controlled “clean room” environments, may settle between the surfaces of the donor and receiver substrate. Imperfect adhesion bonding caused by dust or other particulate can have a pronounced effect, resulting in a drop-out near the point of contact.
Thus, there is a need for an apparatus and method for thermoresist patterning on a substrate using dry media that allows improved edge definition between adhered resist and surrounding areas, that does not direct excessive heat levels onto the donor or receiver substrate, that works well for transfer onto a featured surface, and that is more robust with respect to dust and dirt than is conventional adhesive transfer.